The present invention relates to a method for removing at least nitrogen dioxide (NO2) from carbon dioxide gas comprising NOx and at least one “non-condensable” gas as contaminants. The method typically also removes from the feed gas one or more of at least NO, N2O, water, SOx and CO if present in the gas as further contaminants. The invention has particular application in the purification of crude carbon dioxide, e.g. flue gas generated by oxyfuel combustion of a fuel such as hydrocarbons, carbonaceous fuels and biomass. The method has particular application in treating flue gas from a power station in which pulverized coal is combusted in a boiler to produce steam for electric power generation.
The term “SOx” means oxides of sulfur and includes sulfur dioxide (SO2) and sulfur trioxide (SO3). The term “NOx” means oxides of nitrogen and includes primarily NO and nitrogen dioxide (NO2). NOx may comprise one or more other oxides of nitrogen including N2O, N2O4 and N2O3.
It has been asserted that one of the main causes of global warming is the rise in greenhouse gas contamination in the atmosphere due to anthropological effects. The main greenhouse gas which is being emitted, carbon dioxide (CO2), has risen in concentration in the atmosphere from 270 ppm before the industrial revolution to the current figure of about 378 ppm. Further rises in CO2 concentration are inevitable until CO2 emissions are curbed. The main sources of CO2 emission are fossil fuel fired electric power stations and from petroleum fuelled vehicles.
The use of fossil fuels is necessary in order to continue to produce the quantities of electric power that nations require to sustain their economies and lifestyles. There is, therefore, a need to devise efficient means by which CO2 may be captured from power stations burning fossil fuel so that it can be stored rather than being vented into the atmosphere. Storage may be deep undersea; in a geological formation such as a saline aquifer; or a depleted oil or natural gas formation. Alternatively, the CO2 could be used for enhanced oil recovery (EOR).
The oxyfuel combustion process seeks to mitigate the harmful effects of CO2 emissions by producing a net combustion product gas consisting of CO2 and water vapor by combusting a carbonaceous or hydrocarbon fuel in pure oxygen. This process would result in an absence of nitrogen (N2) in the flue gas, together with a very high combustion temperature which would not be practical in a furnace or boiler. In order to moderate the combustion temperature, part of the total flue gas stream is typically recycled, usually after cooling, back to the burner.
An oxyfuel process for CO2 capture from a pulverized coal-fired power boiler is described in a paper entitled “Oxy-combustion processes for CO2 capture from advanced supercritical PF and NGCC power plants” (Dillon et al; presented at GHGT-7, Vancouver, September 2004), the disclosure of which is incorporated herein by reference.
Oxyfuel combustion produces raw flue gas containing primarily CO2, together with contaminants such as water vapor; “non-condensable” gases, i.e. gases from chemical processes which are not easily condensed by cooling, such as excess combustion oxygen (O2), and/or O2, N2 and argon (Ar) derived from any air leakage into the system; and acid gases such as SO3, SO2, hydrogen chloride (HCl), NO and NO2 produced as oxidation products from components in the fuel or by combination of N2 and O2 at high temperature. The precise concentrations of the gaseous impurities present in the flue gas depend on factors such as on the fuel composition; the level of N2 in the combustor; the combustion temperature; and the design of the burner and furnace.
In general, the final, purified, CO2 product should ideally be produced as a high pressure fluid stream for delivery into a pipeline for transportation to storage or to site of use, e.g. in EOR. The CO2 must be dry to avoid corrosion of, for example, a carbon steel pipeline. The CO2 impurity levels must not jeopardize the integrity of the geological storage site, particularly if the CO2 is to be used for EOR, and the transportation and storage must not infringe international and national treaties and regulations governing the transport and disposal of gas streams.
It is, therefore, necessary to purify the raw flue gas from the boiler or furnace to remove water vapor; SOx; NOx; soluble gaseous impurities such as HCl; and “non-condensable” gases such as O2, N2 and Ar, in order to produce a final CO2 product which will be suitable for storage or use.
In general, the prior art in the area of CO2 capture using the oxyfuel process has up to now concentrated on removal of SOx and NOx upstream of the CO2 compression train in a CO2 recovery and purification system, using current state of the art technology. SOx and NOx removal is based on flue gas desulphurization (FGD) schemes such as scrubbing with limestone slurry followed by air oxidation producing gypsum, and NOx reduction using a variety of techniques such as low NOx burners, over firing or using reducing agents such as ammonia or urea at elevated temperature with or without catalysts. Conventional SOx/NOx removal using desulphurization and NO reduction technologies is disclosed in “Oxyfuel Combustion For Coal-Fired Power Generation With CO2 Capture—Opportunities And Challenges” (Jordal et al; GHGT-7, Vancouver, 2004). Such process could be applied to conventional coal boilers.
WO 2009/010690 A and WO 2009/010691 A disclose a method for the purification of a carbon dioxide feed gas such as flue gas from an oxyfuel combustion process. The method comprises a pre-treatment step (a) to remove at least one of the impurities in the flue gas, a compression step (b) to compress the flue gas, and a carbon dioxide purification step (c) in which pure carbon dioxide liquid is produced by cooling and partial condensing the gas in a cold cycle. The method of WO 2009/010690 A is characterized by a purification step to remove NOx and/or water provided between steps (a) and (c). It is disclosed that the purification step may involve an adsorption system comprising an adsorbent bed which may be regenerated by recycling a portion of the purified gas produced in the adsorption system.
US 2009/0013871 A, US 2009/0013868 A and US 2009/0013717 A disclose a method for purifying crude carbon dioxide feed gas such as flue gas from an oxyfuel combustion system. The feed gas is pre-treated at about atmospheric pressure to remove particulates and/or SOx, compressed and then pre-treated at elevated pressure to remove water and/or NOx. The gas is then subjected to low temperature purification to produce liquid carbon dioxide. The high pressure pre-treatment step may involve adsorption and the gas used to regenerate the adsorption system may be recycled to the oxyfuel combustion system.
EP 0 417 922 A discloses a method for purifying carbon dioxide gas produced, for example, as a gaseous by-product from the production of ammonia or hydrogen, or from fermentation processes. Carbon dioxide feed gas is compressed and dried using a first adsorbent and the dried gas is cooled and distilled into liquid carbon dioxide bottoms and waste overhead. Carbon dioxide is recovered from the waste overhead using a second adsorbent and recycled to the compressor feed. A portion of the carbon dioxide-depleted gas from the second adsorbent is warmed and used to regenerate the first adsorbent before being vented to the atmosphere.
U.S. Pat. No. 5,614,000 A described a combined temperature swing adsorption (TSA)/pressure swing adsorption (PSA) (or “TEPSA”) cycle for operating an adsorption system for removing water and carbon dioxide from air.
U.S. Pat. No. 6,511,640 A, U.S. Pat. No. 5,914,455 A, US 2003/0064014 A and U.S. Pat. No. 5,906,675 A disclose layered adsorption beds for removing impurities such as water, hydrogen (H2), carbon monoxide (CO), carbon dioxide, NO and trace hydrocarbons from gas streams such as air. The beds may comprise an in-bed catalyst for converting CO to carbon dioxide. Preferred catalysts are hopcalite-type catalysts.
US 2003/164092 A discloses a process for removing N2O from a feed gas stream in which the feed gas stream is passed over an adsorbent having a nitrogen diffusion parameter 0.12 s−1 or higher and a N2O capacity of 79 mmol/g/atm or higher at 30° C. Suitable adsorbents include CaX zeolite. The process has particular application in the pre-purification of air to remove water, carbon dioxide and N2O prior to a cryogenic air separation process.
Carbon dioxide-selective permeable membranes are known for recovering carbon dioxide from a vent gas stream. For example, EP 0 410 845 A discloses using a membrane unit to recover gas from stripper overhead vapor in a carbon dioxide liquefaction process. In addition, US 2008/0176174 A discloses using a membrane unit to recover carbon dioxide and oxygen from a non-condensable gas-rich vapor. Flue gas is generated in an oxyfuel combustion system and the vapor is produced in a low temperature process to purify the carbon dioxide from the flue gas. The recovered carbon dioxide and oxygen is recycled to the oxyfuel combustion unit.
There is a continuing need to develop new methods for removing NOx and other contaminants including water, SOx and CO, from carbon dioxide gas, and particularly from crude carbon dioxide gas such as flue gas produced in an oxyfuel combustion process such as that involved in a pulverized coal-fired power boiler.